Human monocytes are activated by endotoxin and other inflammatory microbial products to produce hormone-like factors such as Interleukin 1 (IL 1). IL enhances lymphocytes mitogenic responses to lectins and thus appears to be a nonspecific lymphocyte activating signal that acts jointly with the antigen or polyclonal stimulant to stimulate T cell activation. Partial purification of IL 1 has been accomplished by the use of ultragel AcA 54 column chromatography and affinity chromatography on octyl agarose. Screening of human macrophage cell lines for the production of IL 1 is underway to enable us to produce large quantities of IL 1 for biochemical purification and molecular cloning of the IL 1 gene.